1. Field of the Invention
The present invention relates to a silicone rubber composition wherein the content of low-molecular siloxanes is very low.
2. Description of the Prior Art
Since silicone rubbers have such excellent properties as weather resistance, electrical properties, low-compressive set, heat resistance, and cold resistance, they are used widely in various fields including electronic equipment, automobiles, construction, medicine, and food. For instance, they are used for rubber contacts that are used as rubber contact keys, for example, of remote controllers, typewriters, word processors, computer terminals, and musical instruments; gaskets for construction; various rolls, such as rolls for copying machines, rolls for development, transfer rolls, electrification rolls, and sheet feeding rolls; rubber vibration insulators for audio apparatuses; and packings for compact discs used in computers.
At present, the demand for silicone rubbers is increasing more and more, development of silicone rubbers having excellent properties is desired, and particularly it is strongly demanded to reduce low-molecular siloxanes contained in silicone rubbers.
For example, in the case of rubber contacts used as contact keys, since low-molecular siloxanes cause a contact failure, it is attempted to reduce the amount of low-molecular siloxanes contained in silicone rubber molded products. Specifically, after rubber contact products are cured, the rubber contact products are heat-treated in an oven at 200.degree. C. for 2 to 24 hours to reduce the amount of low-molecular siloxanes, but such heat treatment results in considerable disadvantage in view, for example, of time, labor, and cost.
Further, in the case of roll materials for copying machines, there is the problem that an offset phenomenon occurs, that is, toner is transferred onto a roller through low-molecular siloxanes. Also, for example, in the case of gaskets for construction, it is known that low-molecular siloxanes remaining in the silicone rubber cause difficulties.
Therefore, various techniques for reducing low-molecular siloxanes contained in silicone rubbers are studied.
As a technique of reducing the content of such low-molecular siloxanes, a technique is known wherein an organopolysiloxane having a high degree of polymerization (hereinafter sometimes called simply a high molecular organosiloxane) used as a major component of a silicone rubber is subjected to stripping at a high temperature of 100.degree. to 300.degree. C. under conditions of reduced pressure. However, since the high molecular organosiloxane is high in viscosity, a large amount of energy and much time are required to remove low-molecular siloxnes securely and therefore there is a limit for practical use.
Further, although there is suggested a technique wherein a high molecular organosiloxane is dissolved in a solvent, such as toluene, to remove low-molecular siloxanes by extraction, this technique makes the process complicated and therefore is industrially quite disadvantageous.
Accordingly, a technique of obtaining a high molecular organosiloxane wherein the amount of low-molecular siloxanes is extremely low is demanded.
As a method of producing a high molecular organosiloxane, for example, U.S. Pat. No. 2,546,036 discloses a method wherein sodium hydroxide is used as a catalyst and a silicone oil having a low molecular weight is treated with a polydimethylsiloxane. Further, U.S. Pat. No. 2,634,252 discloses, as a catalyst used in increasing the polymerization rate and in producing a high molecular organosiloxane good in quality and having a high degree of polymerization, potassium hydroxide, a potassium silanolate, a lithium silanolate, a tetraalkylammonium hydroxide, etc.
However, the high molecular organosiloxane obtained by this method contains a considerable amount (generally 5 to 10% by weight) of cyclic low-molecular siloxanes and a large cost is required to remove them.
Further, U.S. Pat. No. 2,883,366 discloses a method of producing a high molecular organosiloxane using a phosphorus siliconate as a polymerization catalyst, but does not refer to the reduction in low-molecular siloxanes at all. Furthermore, a method of producing a high molecular organosiloxane by using a five-coordinate catalyst or the like is studied, but a high molecular organosiloxane having a high degree of polymerization has not yet been obtained stably.